Decoupling mounting plate for tuning fork



F. DOSTAL March 21, 1967 DECOUPLING MOUNTING PLATE FOR TUNING FORK FiledNov. 24, 1965 INVENTOR. 94m Dosma A nmms'r United States Patent3,310,756 DECOUPLNG MOUNTING PLATE FOR TUNING FORK Frank Dostal,Elmhursf, N.Y., assignor to Bulova Watch Company, Inc., New York, N.Y.,a corporation of New York Filed Nov. 24, 1965. Ser. No. 509,510 7Claims. (Cl. 331-116) This invention rel-ates generally to tuning-forkoscillators, and more particularly to an improved mounting plate for atuning fork, which plate has a compliance characteristic effectivelyisolating the fork from the mounting plate with respect to the vibratoryforces, whereby the fork frequency is precisely maintained.

Tuning-fork oscillators have many practical applications, and arecurrently used as frequency standards, for power-line regulation, inguidance systems and geophysical instruments, as tuned filters, and asdriving elements for optical modulators and in timing devices. Theoperating frequency of a vibratory fork is determined by the tinedimensions, the frequency being directly proportional to the thicknessof the tines and inversely proportional to the square of their length.

The utility of a tuningfork oscillator depends on its frequencystability, and it is essential therefore, that the fork operate at aconstant frequency even when the fork is employed in an environmentwhich subjects the fork to external shocks such as in a jet plane, atank, a destroyer, or other moving vehicle. The utility of a tuning-forkoscillator also depends on its operating frequency, and there is agrowing need for forks operating on the high frequency range, that is inthe range 3000 cycles per second.

A tuning fork oscillator is a system constituted by three masses whichare elastically coupled, the first two masses being the tines and thethird being the mounting fixture for the fork. In order for the fork tooperate at a predetermined frequency, the masses and elasticities mustremain constant. There is a tendency, however, particularly inhigh-frequency forks, for spurious oscillations to be excited in themounting plate, as a consequence of which the fork operation, frequencyand stability are adversely affected.

Thus the fork system is effectively altered from a normal systemconstituted 'by three masses and two elasticities, to an abnormal systemhaving an additional elasticity by reason of mounting plate elasticity.The third mass and third elasticity are therefore a source ofdifficulty, for they contribute unknown components to the basicfrequency-determined masses and elasticities.

The tines of a tuning fork vibrate in phase opposition at a frequencydetermined by the parameters of the system. The ends of the tinesexecuted an arcuate motion in the course of vibration, the motionproducing a secondary component of motion which is developed in adirection along the axis of the fork and at a frequency twice that ofthe tines. The reason for this is that the arcuate motion of the tinesincludes an axial vector, and inasmuch as each tine traverses its zeroposition twice in the course of a vibratory cycle, the axial frequencyis double the tine frequency- In a conventional mounting-arrangementwherein the fork is rigidly mounted, the axial component is transmittedto the mounting plate, as a result of which the plate is excited and thestability of the fork is disturbed. The excitation of the plate by othercomponents of fork vibration can be nulled out by carefully balancingthe tines of the fork, but even when the tines are balanced, the axialcomponent will be transmitted to the mounting plate.

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It is known to provide tuning forks with mounting stems resilientlyjoined to the base of the fork or to provide resilient supports for thefork, but while such expedients are effective to decouple the fork fromthe mounting with respect to transversely-directed components of forkmotion, they are ineffective as to the axial component.

Accordingly, the main object of the invention is to provide a mountingplate for a tuning fork, the plate having a compliance characteristicwhich permits the fork to vibrate axially, but which at the same timeacts to effect optimum decoupling of the mounting and the fork withrespect to the axial component. Thus the mounting plate in accordancewith the invention serves to isolate the third mass and third elasticityas to maintain the desired operation, frequency and stability of thefork.

A significant feature of the invention is that it is especiallyeffective in preventing the excitation of suprious vibrations in amounting plate when the fork operates in the high-frequency region. Anadvantage of this feature is that it now becomes possible to makeprecision frequency forksin a range going as high as 25 kilocycles persecond, whereas it was heretofore difficult or impossible to makeprecision forks with frequencies as high as 3 kilocycles.

Briefly stated, these objects are attained in amounting plate having anaperture which is bridged by two bend able beams in parallel relation,the beams being joined by a platform on which is mounted the tuning forkin a manner whereby the tines of the fork lie in a plane parallel to theplane of the mounting plate and the longitudinal axis of the fork isperpendicular to the axis of the beams.

The mass of the fork and the elasticity of the compliance provided bythe bendable beams are such as to establish a frequency which is lessthan twice the fork frequency, whereby the fork is effectivelydecoupledfrom the mounting with respect to the axial component of vibration. Thebeam arrangement is also such as to provide rotational compliance aswell, thereby decoupling the fork and mounting plate with respect tonon-axial components of vibration.

For a better understanding of the invention, as well as other objectsand further features thereof, reference is. made to the followingdetailed description to be read in conjunction with the accompanyingdrawing, wherein:

FIG. 1 is a plan view of a compliant mounting plate in accordance withthe invention; 7

FIG. 2 shows a plug-in type of tuning-fork oscillator, in plan view,with a tuning fork supported on the mounting plate;

FIG. 3 is an end view of the oscillator;

FIG. 4 illustrates in perspective a detail of the drive coil of theoscillator; and

FIG. 5 schematically shows the oscillator circuit.

Referring now to the drawing, there is shown a tuning-fork oscillator inaccordance with the invention, the oscillator including a tuning fork,generally designated by letter T, a mounting plate M therefor, a pickupcoil L a drive coil L and an output coil L The above-listed componentsare protectively housed as an assembly within a casing C provided with amulti-terminal plug P adapted to provide a plug-in connection for thecoils to their associated electronic elements.

In the practical embodiment shown, the tuning fork was designed tooperate at a fundamental mode of 9400 cycles per second, this frequencybeing determinted by the dimensions of tines 10 and 11. As pointed outpreviously, the present invention makes it possible to provide precisiontuning-fork oscillators operating at frequencies as high as 25,000cycles. To minimize the effect of temperature on frequency, the fork ispreferably made of such ferromagnetic alloys .as Ni-Span and Vibraly, toproduce low-temperature coefficients. Alternatively, one may use abi-metallic fork composed of laminae having a positive and negativecoefficient.

The tines 10 and 11 of the fork project upwardly from a base 12 providedwith an internal mounting stud 13 extending midway between the tines.The internal mounting stud serves to mount the fork at its center ofmoment, thereby considerably reducing the effects of shock and vibrationthereon, as well as shortening the over-all length of the fork, .ascompared to an external mounting stud.

The mounting plate M, which may be formed of aluminum, is rectangular inform, and includes a generally rectangular central aperture 14 which isbridged by two bendable beams 15 and 16 disposed in a parallel relation,the beams being interconnected by a transverse platform 17. In practice,the beams and platform may be integral members formed by cuttingsuitable slots in a solid plate to define these elements.

The tuning fork is mounted on the plate by two screws 18 and 19 whichjoin the stud 13 to platform 17, a spacer 20 being interposed betweenthe stud and the platform whereby the tuning fork is disposed in a planeparallel to the plane of the mounting plate, and the tines of the forkare normal to the longitudinal axes of beams 15 and 16.

Mounting plate M is supported on isolation blocks 21 and 22 carried on abase plate 23 resting within the casing C on pillars 24. The blocks areof a material having high flexibility and mechanical damping properties,such as foamedpolyurethane.

Characteristic coil L as best seen in FIG. 4, is an electromagneticelement constituted by two L-shaped polepieces 25 and 26 forming an airgap 27, the pole-pieces being fabricated of soft iron or Permalloy, andbeing joined together by a permanent magnet member 28 surrounded by awinding 29. The air gap 27 is positioned adjacent tine 11, whereby whenthe coil L is energized 'by a pulse of current, the resultant magneticfield acts on the tine to excite vibration therein. Pickup coil L isidentical in design to coil L such that when tine 11 vibrates, itinduces a voltage in the coil. Coil L is also of the same design, but itis disposed adjacent the base of the fork, for reasons to be laterexplained.

As shown in FIG. 5, drive coil L is connected in the emitter-collectorcircuit of a transistor 30 in series with battery 31, the pickup coil Lbeing connected in the base emitter circuit through aresistance-capacitance biasing net work 32 to a transistor amplifierhaving a gain in the order of a few hundred times. The arrangement issuch that the transistor is normally biased to cut-off, but is renderedmomentarily conductive by a voltage induced in pickup coil L by thevibration of tine 11. When the transistor is conductive, a currentimpulse passes through the drive coil L which actuates tine 10, thisprocess being repeated cyclically to maintain the fork in vibration.Thus currents induced in pick-up coil L due to one tine motion areamplified and applied to drive coil L thus maintaining the fork invibration.

The transistor drive arrangement for the fork forms no part of thepresent invention, and the fork may also be maintained in operation byvacuum-tube circuits or other means conventionally used for driving atuning fork.

When the tines 10 and 11 of the fork vibrate along an arcuate path, anaxial component of motion is produced which is transmitted toward thebase 12 along the axis extending along stud 13 and represented by line Xin FIG. 5. In the event the mounting of the fork were rigid, this axialmotion would in turn be transmitted to the mounting plate, with theadverse effect previously noted. However, the fork is supported on beams15 and 16, which impart axial compliance to the mounting along axis X.The degree of compliance provided by the elasticity of the beams is madesuch with respect to the mass of the fork supported thereon as toestablish a frequency which less than twice the fundamental frequency ofthe fork. Hence, the frequency of the compliant mounting is non-resonantwith respect to that 'of the fork, and the fork and the mounting areeffectively decoupled in regard to the axial component of fork motion.Because of the difference in the frequency of the fork and the frequencycharacteristic of the mounting, the vibration of the fork in the axialdirection does not act to produce parasitic oscillations in themounting.

The compliant mounting is also capable of rotation about the point ofintersection between an axis Y perpendicular to the X-axis and midwaybetween beams 15 and 16, so as to decouple the fork from the mountingwith respect to non-axial components of fork motion which are nototherwise completely balanced out.

Thus the efficiency of the fork is maintained at its optimum value andits stability is not adversely affected by the mounting therefor, nor byexternal shocks, for the fork is also isolated from such shock. While itis sufficient that the resonance of the compliant mounting, with thefork in place, be under twice the fork frequency, the stiffness must bemaintained sufficiently high so that external vibrations, such as thatproduced by jet aircraft, induced along the tine axis do not themselvesexcite resonance.

It will be noted that the oscillator has two outputs, the first of whichOUTI is derived from pick-up coil L and has a stable frequencydetermined by the tine frequency, and the second of which OUT-II isderived from coil L and is twice the tine frequency. As pointed outpreviously, the base of the fork has a vibratory component transmittedthereto which is double the tine frequency and it is this harmoniccomponent which is picked up by cOil L3.

While there has been shown a preferred embodiment of mounting plate fortuning fork in accordance with the invention, it will be appreciatedthat many changes and modifications may be made therein without,however, departing from the essential spirit of the invention as definedin the annexed claims.

What is claimed is:

1. A precision tuning-fork oscillator comprising:

(a) a tun-ing fork having a pair of tines interconnected by a base andextending therefrom in \a common plane, said base having a mounting studprojecting therefrom,

(b) means to sustain said fork in vibration at the natural frequency ofsaid tines, the motion of said tines producing an axial component ofmotion which is transmitted to said stud,

(c) a mounting plate for supporting said fork, said plate having a rigidportion for securing-said plate to a fixture and a compliant portionresiliently connected to said rigid portion,

(d) and means securing the stud of said fork to said compliant portionto dispose the plane containing said tines in parallel relationship tothe plane of said mounting plate, the compliant portion of said platehaving a value of elasticity which in combination with the mass of saidfork establishes a frequency which is less than twice the naturalfrequency of the fork.

2. An oscillator as set forth in claim 1, wherein said stud projectsupwardly from said base between said tines to provide a relatively shortfork.

3. An oscillator as set forth in claim 1, wherein said means to sustainsaid fork in vibration includes a fixed drive coil operatively coupledto one of said tines, a fixed pickup coil operatively coupled to theother of said tines, and a transistor circuit whose input is coupled tothe pickup coil and whose output is coupled to the drive coil to applypulses thereto under the control of voltages induced in said pickupcoil.

4. An oscillator as set forth in claim 1, including a fixed output coiloperatively coupled to the base of said fork to produce an outputvoltage whose frequency is twice the natural fork frequency.

5. An oscillator as set forth in claim 1, wherein said mounting-platecompliant portion is formed by an aperutre which is bridged by a pair ofbendable beams in parallel relationship, the beams being interconnectedat their intermediate pontions by a platform, said fork studbeingsecured to said platform.

6. An oscillator as set forth in claim 5, wherein said beams'andplatforms are integral members formed by slots cut into a solid plate.

7. In a precision tuning fork oscillator, the combination of a tuningfork having a pair of tines extending from abase provided with aninternal mounting stud, and a mounting plate for said fork whichincludes a pair of bendable beams bridging a central aperture in saidplate and a platform transversely spanning said beams, said stud beingattached to said platform to support said fork in a plane parallel tosaid plate, the compliance afforded by said beams having an elasticitywhich in combination with the mass of said fork establishes a frequencylower than the natural frequency of the fork.

No references cited.

ROY LAKE, Primary Examiner.

I. KOMINSKI, Assistant Examiner.

1. A PRECISION TUNING-FORK OSCILLATOR COMPRISING: (A) A TUNING FORKHAVING A PAIR OF TINES INTERCONNECTED BY A BASE AND EXTENDING THEREFROMIN A COMMON PLANE, SAID BASE HAVING A MOUNTING STUD PROJECTINGTHEREFROM, (B) MEANS TO SUSTAIN SAID FORK IN VIBRATION AT THE NATURALFREQUENCY OF SAID TINES, THE MOTION OF SAID TINES PRODUCING AN AXIALCOMPONENT OF MOTION WHICH IS TRANSMITTED TO SAID STUD, (C) A MOUNTINGPLATE FOR SUPPORTING SAID FORK, SAID PLATE HAVING A RIGID PORTION FORSECURING SAID PLATE TO A FIXTURE AND A COMPLAINT PORTION RESILIENTLYCONNECTED TO SAID RIGID PORTION, (D) AND MEANS SECURING THE STUD OF SAIDFORK TO SAID COMPLAINT PORTION TO DISPOSE THE PLANE CONTAINING SAIDTINES IN PARALLEL RELATIONSHIP TO THE PLANE OF SAID MOUNTING PLATE, THECOMPLAINT PORTION OF SAID PLATE HAVING A VALUE OF ELASTICITY WHICH INCOMBINATION WITH THE MASS OF SAID FORK ESTABLISHES A FREQUENCY WHICH ISLESS THAN TWICE THE NATURAL FREQUENCY OF THE FORK.